1. Field of the Invention
The present invention relates to a lead frame and a method of manufacturing the same and, more particularly, a lead frame employed in a leadless package such as QFN, SON, etc. and a method of manufacturing the same.
2. Description of the Related Art
In recent years, the development of the LSI technology, which is the key technology to implement the multimedia devices, has advanced steadily toward the higher speed and the larger capacity of the data transmission. According to this, the higher density of the packaging technology as the interface between the LSI and the electronic device is advanced.
As the package that can accommodate the high density packaging, various packages have been developed. As the package for the high density packaging using the lead frame, for example, the leadless packages such as QFN (Quad Flat Non-leaded Package), SON (Small Outline Non-leaded Package), etc., in which the leads are not extended to the out side, have been known.
FIG. 1A is a sectional view showing an example of a semiconductor device in which an IC chip is installed into the leadless package, and FIG. 1B is an enlarged sectional view taken along a IIxe2x80x94II line in FIG. 1A. FIG. 2A is a plan view showing an example of a lead portion of the lead frame employed in the leadless package, and FIG. 2B is a sectional view taken along a IIIxe2x80x94III line in FIG. 2A.
As shown in FIG. 1A, in a semiconductor device 110, an IC chip 104 is mounted on a die pad 102 and connection electrodes (not shown) of the IC chip 104 are connected electrically to lead portions 100 via wires. Then, the IC chip 104 is sealed with a mold resin 106 to cover up to the top end portion of the lead portions 100.
In this manner, in the semiconductor device 110 having the QFN structure, upper surfaces and side surfaces of the lead portions 100 are covered with the mold resin 106. Therefore, if a cross section of the lead portion 100 is a perpendicular shape, for example, it is possible that the lead portions 100 buried in the mold resin 106 come off the mold resin 106.
Therefore, as shown in FIGS. 2A and 2B, a sectional structure of the lead portion 100 of the lead frame is formed like the inverse-taper shape, two-stepped shape, etc., for example, in which a width of a surface (a surface that is connected to the connection electrode of the IC chip) is thicker than a width of a back side (a surface that is connected to the wiring board, etc.).
As a result, as shown in FIG. 1B, the mold resin 106 can be buried to cut into side surfaces of the lead portions 100. Thus, the lead portions 100 can be prevented from coming off the mold resin 106.
In the prior art, in order to form the lead portions 100 having the above sectional structure, first almost same patterns of the resist films are formed on both surfaces of the metal plate. Then, predetermined portions of the metal plate are etched from both surfaces by the wet etching using the chemicals, etc. while using the patterns of the resist films as a mask. At this time, if the etching is not applied to the surface and the back side of the metal plate under the same condition and, for example, the etching is applied excessively to the back side, the lead portion whose width on the surface is thicker than a width on the back side can be formed.
As described above, in the prior art, in order to form the inverse taper shape, etc. simply, the lead frame employed in the leadless package is mainly manufactured by the wet etching.
However, the method of working the lead frame by the wet etching has the fatal defect such that the cost is high and the working speed is slow. For this reason, the method of manufacturing the lead frame having the lead portions of the above sectional structure such as the inverse-taper shape, the two-stepped shape, etc. by applying the punching, etc. to the metal plate by means of the precision stamping die, which makes possible the low cost and the high working speed, is expected earnestly.
It is an object of the present invention to provide a lead frame that can be manufactured by the stamping to use the precision stamping die and can be employed in a leadless package, and a method of manufacturing the same.
The present invention provides a lead frame which comprises lead portions extended from a frame portion toward an inside like a teeth of a comb, the lead portions including a top end portion, a center portion, and a base portion connected to the frame portion; wherein the top end portion and the center portion are connected via a first constriction portion, and the center portion and the base portion are connected via a second constriction portion, and thickness of both side surface portions of the center portion and the top end portion of the lead portions is set thinner than thickness of remaining portions of the lead portions.
The present invention improves the structure of the lead portion so as to make it possible to manufacture the lead frame that can be fitted to the leadless package whose lead portions are not extended to the outside of the sealing resin, i.e., the lead frame in which a sectional structure of the lead portion is formed like the inverse-taper shape, two-stepped shape, etc., by the stamping to use the precision stamping die without the disadvantage.
As the result of earnest study of the method that manufactures the above lead frame by the precision stamping die, the inventors of the present invention found the problems described in the following.
In order to manufacture the lead frame having the lead portions a sectional structure of which is formed like the inverse-taper shape, two-stepped shape, etc. by the stamping to use the precision stamping die, as shown in FIG. 3A, first original forms of lead portions 10 are formed by punching predetermined portions of a metal plate vertically sequentially by a predetermined punch of a die. At this time, the lead portion 10 comprises a main portion 10a and a base portion 10b connected to a frame portion 10c, and the main portion 10a is formed to have a larger width than a width of the base portion 10b. 
Then, as shown in FIG. 3C, side surface portions 10d of the main portion 10a of the lead portion 10 is pushed by a side-surface portion punch 12 to reduce a thickness, so that a two-stepped shape is formed. At this time, the so-called burr is ready to occur on the almost right-angle portion (C portion) at which a width is expanded from the base portion 10b of the lead portion 10 to the main portion 10a, due to the displacement of the side-surface portion punch 12.
Then, as shown in FIG. 3E, a top end portion 10e of the main portion 10a of the lead portion 10 is pushed by a top-end portion punch 12a to reduce a thickness, so that a two-stepped shape is formed. At this time, the burr is ready to occur on the portion (D portion) at which the portion, which is pushed by the top-end portion punch 12a, of the main portion 10a of the lead portion 10 intersects with the portion pushed by the side-surface portion punch 12. In this case, the reason why the side surface portions of the main portion and the top end portion of the lead portion are pushed by separate punches is that the integral formation of the punch that can push both the side surface portions and the top end portion collectively is difficult from the aspect of the strength of the punch member.
The present invention is invented based on the generation of the above disadvantage when the side surface portions and the top end portion of the lead portion are formed to have the two-stepped shape by the stamping to use the precision stamping die. In other words, the lead portion comprises the top end portion, the center portion, and the base portion, and side surface portions of the center portion and the top end portion of the lead portion are connected via a first constriction portion, and the side surface portions of the center portion and the base portion of the lead portion are connected via a second constriction portion.
Therefore, in order to shape the side surface portions of the center portion of the lead portion into the two-stepped shape, first the almost right-angle portion mentioned as above can be eliminated when the side surface portions of the center portion of the lead portion are pushed by the side-surface portion punch of the die. Thus, generation of the burr can be prevented. Then, the top end portion of the lead portion and the side surface portions of the center portion are connected via the first constriction portion when the top end portion of the lead portion is pushed by the top-end portion punch. Therefore, the portion at which the portion pushed by the top-end portion punch intersects with the side surface portions of the center portion pushed by the side-surface portion punch can be eliminated. As a result, generation of the burr can also be prevented.